In production of automobile components that require a high fatigue strength and a high wear resistance, treatments, such as an induction hardening treatment and a soft-nitriding treatment that are a casehardening treatment, are carried out after forging and machining in most cases.
The “soft-nitriding treatment” performs a cementation treatment on nitrogen and carbon at a temperature of an A1 transformation point or less, and has such major characteristics that have a low heat-treatment temperature and a smaller heat treatment strain than that in the “induction hardening treatment”. A “compound layer” (layer formed of precipitated nitride such as Fe3N) observed as a white portion through etching using nital is formed in a surface layer of the component subjected to the soft-nitriding treatment. A “diffusion layer” is formed between the above compound layer and a base metal (base material).
The soft-nitriding treatment causes a small heat treatment strain, but cannot eliminate this strain, and thus brings not a small bad influence on dimensional accuracy. Particularly, even a slight deterioration of dimensional accuracy becomes a crucial matter in a crankshaft or the like that is a rotational shaft component. Hence, it is required to perform bending-straightening after the soft-nitriding treatment so as to improve the dimensional accuracy.
Unfortunately, cracks may be generated from the surface layer if the soft-nitrided component is subjected to the bending-straightening. Hence, a soft-nitrided component, such as a crankshaft, is required to experience no cracks even if being subjected to bending-straightening, that is, to have an excellent bending straightening property as well as a high bending fatigue strength.
In the following description, the soft-nitrided component may be represented by a crankshaft in some cases.
Because of current demand for consideration to the environments, a crankshaft that is a major component of an engine is also oriented to reduction in weight and size without exclusion, and has been required to have an extremely high bending fatigue strength of 750 MPa or more, for example.
In the light of cost reduction, resource saving, and others, there has been increased demand for a non-thermal refined crankshaft without being subjected to a “quenching-tempering treatment” (thermal refining treatment) during the production thereof.
In order to secure the above bending fatigue strength of 750 MPa or more in a non-thermal refined crankshaft, it is required to set hardness at a position of 0.05 mm from the surface of the component (also referred to as a “surface-layer hardness”, hereinafter) to be at least 400 or more in terms of a Vickers hardness (referred to as a “HV hardness”, hereinafter) after the soft-nitriding treatment.
However, in the case of setting the HV hardness at a position of 0.05 mm from the surface of the crankshaft to be 400 or more, cracks are likely to be generated in the surface layer if the bending straightening is performed. Conducting a bending fatigue test on such a crankshaft results in fatigue fractures initiated from the above cracks.
In addition, as described above, there has been increased demand for further reduction in weight of a crankshaft, and thus further more flexibility has been required in crankshaft shape designing. Consequently, steel material for a crankshaft is required to have a bending straightening property high enough for a crankshaft having a shape likely to exhibit a greater bending than that in a conventional art during the soft-nitriding to be bending-straightened.
Accordingly, there has been extremely strong demand for a crankshaft having a sufficient bending straightening property in addition to a bending fatigue strength as high as 750 MPa or more.
To meet the above demand, for example, Patent Document 1 discloses a “non-thermal refined steel for soft-nitriding”, wherein the steel contains, in mass %, C: 0.2 to 0.6%, Si: 0.05 to 1.0%, Mn: 0.25 to 1.0%, S: 0.03 to 0.2%, Cr: 0.2% or less, s-Al: 0.045% or less, Ti: 0.002 to 0.010%, N: 0.005 to 0.025%, and 0: 0.001 to 0.005%, and further contains one or more types of elements selected from Pb: 0.01 to 0.40%, Ca: 0.0005 to 0.0050%, and Bi: 0.005 to 0.40% if necessary, satisfies conditions: 0.12×Ti %<O %<2.5×Ti %, and 0.04×N %<O %<0.7×N %, and includes a balance made of Fe and unavoidable impurities, wherein a micro-structure after hot forging is a mixed structure of ferrite and perlite.
Patent Document 2 discloses a “crankshaft” made of a steel whose surface is subjected to a nitriding treatment or a soft-nitriding treatment, the crankshaft including a pin section and a journal section, wherein the steel contains, as an alloy element, C: 0.07 mass % or more to 0.12 mass % or less, Si: 0.05 mass % or more to 0.25 mass % or less, Mn: 0.1 mass % or more to 0.5 mass % or less, Cu: 0.8 mass % or more to 1.5 mass % or less, Ni: 2.4 mass % or more to 4.5 mass % or less, Al: 0.8 mass % or more to 1.5 mass % or less, Ti: 0.5 mass % or more to 1.5 mass % or less, and further contains one or more types of elements selected from S: 0.01 mass % or more to 0.10 mass %, Ca: 0.0010 mass % or more to 0.0050 mass % if necessary, and includes a balance made of Fe and unavoidable impurities; and each steel specimen taken from a center part of the steel that is affected by no influence of the nitriding treatment is subjected to a solid solution treatment at 1200° C. for one hour, and thereafter, is cooled at an appropriate cooling speed of 0.3° C./seconds or more to 1.5° C./seconds or less within a temperature range from 900° C. or more to 300° C. or less, thereby setting a ratio of bainite in the steel micro-structure to be 80% or more, and setting the HV hardness to be 200 or more to 300 or less; each internal hardness of the pin section and the journal section that are subjected to the nitriding treatment or the soft-nitriding treatment is set to be 350 or more to 500 or less in terms of the HV hardness; and the HV hardness at a position of 0.05 mm from the surface is 650 or more to 950 or less.
In Patent Document 3, the present inventors have proposed a “non-thermal refined nitrided crankshaft” wherein a steel material of a base metal contains, in mass %, C: 0.25 to 0.60%, Si: 0.10 to 1.0%, Mn: 0.60 to 2.0%, P: 0.08% or less, S: 0.10% or less, Al: 0.05% or less, Cr: 0.20 to 1.0%, and N: 0.0030 to 0.0250%, includes a balance made of Fe and impurities, and satisfies 40-C+2Mn+5.5Cr≤43.0; and the HV hardness at a depth of 0.05 mm from the surface is 380 to 600, and at least each of a pin fillet section, a journal fillet section, and a pin section has a compound-layer depth of 5 μm or less.
This non-thermal refined nitrided crankshaft may further contain one or more types of elements selected from Cu, Ni, Mo, V, Ti, and Ca, and in this case, it is necessary to satisfy [40-C+2Mn+5.5Cr+26Mo≥43.0].
In Patent Document 4, the present inventors have further proposed a “thermal refined soft-nitrided component”, wherein a steel material of a base metal contains, in mass %, C: 0.25 to 0.40%, Si: 0.10 to 0.35%, Mn: 0.60 to 1.0%, P: 0.08% or less, S: 0.10% or less, Al: 0.05% or less, Cr: 0.30 to 1.10%, and N: 0.0030 to 0.0250%, and includes a balance made of Fe and impurities; and the HV hardness at a position of 0.05 mm from the surface is 400 to 600, and a compound-layer depth at a stress concentrated region is 5 μm or less.
The thermal refined nitrided component may further contain one or more types of elements selected from Cu, Mo, V, Ni, and Ti.